1. Field of the Invention
The invention relates to a method of preparing alkyloligoglycosides by glycoside alcoholysis of lower alkylglycosides with alcohols.
2. Discussion of the Background
Alkyloligoglycosides can be prepared entirely or partly based on renewable raw materials. These compounds command great interest because of their low toxicity, their very good biological degradability, and their interesting practical properties. According to F. A. Hughes and B. W. Lew, J.A.O.C.S. 47 (1970), 162, the properties of the alkyloligoglycosides are strongly determined by their degree of oligomerization. For this reason there is interest in processes by which alkyloligoglycosides with definite average degrees of oligomerization can be selectively prepared.
Glucose can be converted smoothly to an alkylglucoside by the Fischer glycoside synthesis with hydrophilic alcohols such as methanol or ethanol and acid catalysis. With hydrophobic alcohols, on the other hand, solubility problems occur.
According to U.S. Pat. No. 3,219,656, higher alkylglucosides are obtained by first preparing butylglucoside, which is then subjected to alcoholysis with a higher alcohol. This process requires large amounts of cation exchanger, however.
German OS No. 19 43 689 discloses that higher alkyloligosaccharides can be prepared from butylglucoside by transacetalization with alcohols containing 11 to 32 carbon atoms and acid catalysis. The degree of oligomerization is inversely proportional to the moles of alcohol used per mole of butylglucoside.
This reciprocal relationship is also confirmed in German No. OS 19 05 523. The alcohol-monosaccharide molar ratios in the transacetalization in this case are set at 0.05 to 12, whereby alkyloligoglucosides are obtained with an average of 25 to 1.4 glucose units per molecule.
Accordingly, the alcohol-carbohydrate ratios chosen for high degrees of oligomerization must be so low that the course of the reaction is not impaired by the formation of viscous products. With low degrees of oligomerization, on the other hand, the process is carried out with very large excesses of alcohol that must be distilled off after completion of the reaction. Because of the very diverse amounts of alcohol which are required, a continuous method of production suitable for products having varying degrees of oligomerization from 1.5 to 5, for example, in a single system can only be realized at great expense.
According to U.S. Pat. No. 3,839,318 and 4,465,828, the acid added as catalyst affects only the rate of reaction. No effect of the catalyst concentration on the degree of oligomerization is disclosed. Further, this effect is not expected since it was stated in U.S. Pat. No. 4,223,129 that the catalyst concentration has no effect on the degree of oligomerization in the preparation of methyloligoglycosides.
A need exists therefore for a method of preparing alkyloligoglycosides which is easily adjusted and which reproducibly produces specific degrees of oligomerization.